Developmental differences in the analgesia produced by the central cholinergic system.

In order to study the ontogenesis of cholinergically mediated analgesia in the central nervous system (CNS), 10-day-, 28-day-, and 3-month-old rats were injected with .025, .05, and .10 mg/kg of oxotremorine, a muscarinic receptor agonist. To restrict the effect of oxotremorine to the CNS, methylscopolamine, a peripheral muscarinic antagonist, was injected simultaneously (.19 mg/kg, a dose equimolar to .10 mg/kg of oxotremorine). Following drug injections the tail-flick procedure was used to assess analgesia. Results revealed that oxotremorine was completely ineffective in producing analgesia in the 10-day- and 28-day-old age groups. By 3 months of age oxotremorine produced a dose-dependent analgesia. Since most neurochemical markers of cholinergic receptor function are at mature or near mature levels by 28 days of age, neurochemical indexes of receptor function overestimate the analgesic function of the central cholinergic system.